The present invention is related to the field of circuits utilized for monitoring the output of a sensor device. More specifically, the present invention is related to such sensor circuits which utilize an integrated circuit, and which preferably monitor the output of a sensor, such as a reluctance sensor, which senses engine crankshaft position of an internal combustion engine.
In prior internal combustion engine control systems, reluctance sensors are utilized to sense the passage of projections rotated synchronously with the engine crankshaft. Typically either reluctance or Hall effect sensors are utilized, and the output of these sensors must be suitably amplified to produce appropriate signals for further processing by an engine control module. Advantageously, the first circuit which receives the output of the sensor device is an integrated circuit (IC) such as Motorola, Inc. sense amplifier MC1440 or National Semiconductor voltage comparator/buffer LM111. The aforesaid integrated circuits essentially utilize a differential common emitter connected input stage to sense the output of a magnetic position sensor such as a reluctance sensor or a Hall effect device. However, these prior sense amplifiers typically cannot tolerate input DC levels which fall below the bias potential applied to the integrated circuit substrate base. This is because typically the integrated circuit substrate base must be maintained at the lowest possible voltage potential in order to prevent substrate injection which corresponds to forward biasing of the integrated circuit isolation junction between the IC substrate base and the epitaxial layer grown on top of the integrated circuit substrate base. The problem of substrate injection is discussed in U.S. Pat. No. 4,574,221 to Hess et al, assigned to the assignee of the present invention, in the context of preventing substrate injection due to transients caused by a large inductive or capacitive output loads. However, conventional integrated circuit sense amplifiers do not permit operation of either of their input terminals at potentials below the substrate bias potential since this would tend to produce substrate injection.
With prior integrated circuit sense amplifiers, in order to sense below ground input voltages, typically positive and negative power supplies must be utilized for supplying operative power to the sense amplifier. Then the IC substrate base is maintained at the lowest possible negative voltage potential. The requirement for both positive and negative power supplies adds to the expense of utilizing such sense amplifiers if it is necessary to operate at input signals which can vary both above and below ground potential.